The possibilities are quite enticing, no? Having said that not stopping by Union would be problematical for ridership.
AoD
Through Union and up the (hopefully) future Via HFR corridor, with a short spur off of it to connect to the airport? An airport express train connecting to the CBD and both airports forming a giant elongated U.
EDIT: If I'm not mistaken, the semi-proposed Via HFR corridor goes right through the Pickering lands, no?